A TUBULARIAN HYDROMEDUSAN 167 
communication with one another. When a colony is well fed 
it grows rapidly, new hydranths bud out from the stalk, and 
medusoid buds appear and grow into meduse. The hydranths 
are frequently destroyed by frost, or by the beating of waves, 
or by fishes, but new ones quickly grow in their places. 
The medusoid stage. The medusoids of tubularian hydro- 
medusans are either sessile sporosacs or free-swimming meduse. 
Pennaria produces both kinds. During a greater part and 
in some cases the whole of the year they are sporosacs, but 
usually during the middle and last of the summer they detach 
themselves from the hydroid and become meduse. Both con- 
ditions may be found in the same colony and at the same time. 
The medusoids of Bougainvillea are always free-swimming and 
with other medusz will be found in the surface waters of the 
ocean. They may be easily obtained by placing the live hydroid 
colony in a small glass of sea water; the medusz will be found 
swimming about in the water. 
Place several meduse of Bougainvillea or of any other tubu- 
larian hydromedusan in a watch-glass of sea water or, if they 
are preserved specimens, in alcohol. If they are alive, observe 
the swimming motions. Note the radiate type of structure. 
The body is bell-shaped, having an outer convex and an inner 
concave side, and on its margin are tentacles (in the medusa of 
Pennaria they are rudimentary). The convex side is called the 
exumbrella or aboral side, and the concave, the subumbrella or oral 
side. In the center of the latter is the proboscis-like projection 
called the manubrium, at the distal end of which is the mouth, 
surrounded by short oral tentacles. ‘The mouth opens into the 
gastro-vascular space, which comprises the space within the manu- 
brium and also a system of canals in the bell-shaped body. 
These canals consist of four radial tubes, which extend from 
the base of the manubrium to the periphery of the body, and 
are there united by a circular tube, which runs parallel with the 
margin and close to it. 
